Radio-frequency identification (RFID) is a technology that uses communication via radio frequency (RF) waves to exchange data between a “reader” (or “interrogator”) and an electronic RFID “tag” (or “transponder”) which is attached to (or otherwise associated with) an object being monitored (OBM) (or “tracked object”), usually for purposes such as identification and tracking. The tag may interface with a sensor that produces information such as a sensor reading relating to the tracked object. Either or both of the RFID tag and its sensor may be at least partially embedded in the tracked object.
RFID tags generally comprise at least two parts, namely: an integrated circuit (IC) for storing and processing information, modulating and demodulating a radio-frequency (RF) signal and other specialized functions, and an antenna (ANT) for receiving and transmitting and receiving signals, such as from an external reader (or interrogator). Generally, at least the IC portion of the tag may be enclosed in some kind of housing.
There are two main types of RFID tags: passive and active. Active RFID tags are also referred to as “battery assisted”.
Passive RFID tags have no power source (no battery) and require an electromagnetic field from an external source (such as the reader) to power the tag electronics and initiate a signal transmission. In the context of a passive tag, “transmission” may mean modulating an impedance or resonance of an antenna, such as simply shorting or not shorting the antenna, resulting in “backscatter”. These modulations of the antenna can be sensed by the external reader. An antenna may be a coil in a low frequency (LF) or high frequency (HF) magnetic field coupled system or an ultra-high frequency (UHF) dipole in an electric field coupled system. Passive RFID tags can also energize a sensor circuit, when power is being supplied to the tag by the external reader. Passive RFID tags in the LF or HF range often require powerful readers because of powering limitations of magnetic field coupling which falls off dramatically. The coupling corresponds to 1/r3, with 1 being signal power and r being distance between tag and reader.
Battery assisted RFID tags, on the other hand, include a battery and a transmitter, and can transmit signals to an external reader. In contrast to the passive RFID tags, battery assisted RFID tags derive all their circuit power from the battery. Similar to passive RFID, communication is still achieved using the backscatter method described above. The range of battery assisted RFID typically exceeds that of passive RFID because the power needed from the reader is only what is necessary for the communication link. Battery power is also used to provide a stable supply for sensor sampling. Battery assisted devices that are RFID enabled can therefore make measurements, such as temperature, independently of the reader. The transmissions may occur at periodic intervals, independent of whether there is an external reader nearby (since the reader is not needed to power the active circuit), or the tag may transmit in response to a query (request for the tag to transmit) by the external reader.
In various applications, it is useful to take measurements on an object to be measured (OBM) at periodic intervals such as daily, weekly, monthly, quarterly, bi-annually or annually. This is the case for health monitoring of many types of structures, for example pipelines, bridges, ships, aircraft, oil rigs, oil storage tanks, buildings and the like.
Often, parts of the OBM are not readily accessible after installation (for example: buried foundations, under water, inside tanks, etc.) or are otherwise difficult or inconvenient to measure. Therefore, it is desirable to provide a wireless interface with the measurement sensor circuitry. This allows measurements to be taken without requiring physical contact with the sensor, which may not be possible due to the location or accessibility of the OBM.
However, in applications in which sensed measurements are transmitted wirelessly, it has proven difficult to provide an on-board power supply that can power the sensor and its associated circuitry such as a transponder over the entire lifespan of the OBM. A first reason for this is that it may not be possible to access the sensor and/or the transponder circuitry so as to recharge or replace a faulty or depleted power supply. A second reason for this is that the OBM may have a compact or flat profile, and therefore it may be undesirable to add a bulky power supply to the otherwise compact sensor circuitry. Further disadvantages of batteries for this type of application are also known; for example, periodic power draws on batteries with long intervals of “down time” during which no power is drawn can drastically reduce the performance of certain types of batteries.
As a consequence, it is known to provide a wireless sensor such as an RFID sensor that is passively powered. Passive RFID tags collect energy from a nearby RFID reader's interrogating radio waves. The RFID tag uses collected energy to perform operations such as taking measurements from a sensor and transmitting those measurements to the external RFID reader. In this way, an RFID sensor can be provided that does not require an on-board battery, so that the disadvantages associated with batteries may be avoided.
U.S. Pat. No. 9,378,448 “RFID Sensor Tag and System for Small Output Transducers, and Related Methods”, the entire disclosure of which is incorporated herein by reference, describes a passive RFID tag may be provided to measure a strain. Using passive RFID, such measurements can be periodically conducted over long time periods, such as the lifespan of the associated structure.